FOV = 2*arctan(0.5*width/distance)
Where w is the width of the linear field at a distance d (don't forget to get them in the same usnits ... the US form is feet per 1000 yards). This is the exact solution.
For most bin calculations to two significant digits this one works OK
FOV = arctan(width/distance)
and you can do it at Google (with unit conversion) try a search on
arctan(430/3000) in degrees
http://www.google.com/search?&q=arctan(430/3000)+in+degrees
to see what I mean.